Pneumatic screwdriver is a tool for manually securing or loosening a screw, and a traditional pneumatic screwdriver as disclosed in R.O.C. Pat. Publication No. 167986 includes a pneumatic motor for driving a power axle to push a transmission seat, and the transmission seat includes a plurality of steel ball holes for passing a cam rod, and the cam rod has a steel ball seat, and the steel ball seat includes a plurality of steel ball grooves, and a steel ball is placed between the steel ball groove and the steel ball hole. A torque spring presses on the transmission seat and the steel ball, so that the transmission seat and the cam rod form a transmitting relation, and the cam rod at its end portion has a hexagonal hole for installing various different disassembling tools such as a slotted screwdriver or a Philips screwdriver. With the transmitting relation, users can enjoy the tremendous convenience brought by the pneumatic tool. The detailed structure of a transmission seat in the foregoing prior art pneumatic screwdriver as shown in FIGS. 1A and 1B includes a through hole 11, and the through hole 11 at its periphery has a plurality of steel ball holes 12, each for installing a steel ball 13, and the sidewall of the steel ball hole 12 has leaning portions 141, 142. If the pneumatic screwdriver is turned clockwise or counterclockwise, the steel balls 13 press the leaning portions 141, 142. If a user operates the pneumatic screwdriver to secure or loosen a screw, the rotating torque drives the cam rod to produce a reacting force. If the reacting force is greater than the pressing force of a torque spring, then the steel ball will push the torque spring, and the transmission seat 10, steel ball 13, and steel ball groove will produce a continuous sliding action. Now, the pneumatic screwdriver reaches a predetermined torque, so that the steel ball seat, transmission seat 10, and steel ball 13 slide with each other to output a predetermined torque instead of applying a larger torque. Since the prior art leaning portions 141, 142 are parallel with each other, therefore after the steel ball 13 regardless of being turned clockwise or counterclockwise reaches a specific torque, the steel ball will be separated from the steel ball groove, so that the torques for securing and loosening a screw are consistent. If it is necessary to loosen a larger screw or secure a tighter screw, the pneumatic screwdriver must be taken apart and a tool (such as a wrench) must be used to tighten the torque spring to increase the torque of the pneumatic screwdriver, and such procedure wastes time and efforts. Furthermore, users have to repeat the abovementioned operating procedure if the torque of the pneumatic screwdriver is not sufficiently adjusted, and thus causing tremendous inconvenience to users.